Various modulation techniques have been implemented to transmit digital information. For example, orthogonal frequency division multiplexing (OFDM), which spreads data to be transmitted over a large number of carriers, e.g., more than a thousand carriers, has been utilized to transmit digital information. In a system that implements OFDM modulation, the modulation symbols on each of the carriers are arranged to occur simultaneously and the carriers have a common frequency spacing, which is the inverse of the duration, called the active symbol period, over which a receiver will examine a received signal and perform the demodulation. In general, the carrier spacing ensures orthogonality of the carriers. That is, the demodulator for one carrier does not see the modulation of the other carriers in order to avoid crosstalk between carriers.
A further modulation refinement includes the concept of a guard interval. That is, each modulation symbol is transmitted for a total symbol period which is shorter than the active symbol period by a period known as the guard interval. This is employed so that the receiver experiences neither inter-symbol nor inter-carrier interference, provided that any echoes present in the signal have a delay which does not exceed the guard interval. Unfortunately, the addition of the guard interval reduces the data capacity by an amount dependent on the length of the guard interval. With OFDM it is generally possible to protect against echoes with prolonged delay by choosing a sufficient number of carriers that the guard interval need not form too great a fraction of the active symbol period. In general, the complex process of modulating (and demodulating) thousands of carriers simultaneously is equivalent to performing discrete Fourier Transform operations, for which efficient Fast Fourier Transform (FFT) algorithms exist. Thus, integrated circuit (IC) implementations of OFDM demodulators are feasible for affordable mass-produced receivers. However, uncoded OFDM is generally not satisfactory with selective channels. As such, a number of communication systems have implemented Coded Orthogonal Frequency Division Multiplexing (COFDM).
COFDM has been used for various digital broadcasting systems and is particularly tolerant to the effects of multipath, assuming a suitable guard interval is implemented. More particularly, COFDM is not limited to ‘natural’ multipath as it can also be used in so-called Single-Frequency Networks (SFNs). As is well known, a SFN includes multiple transmitters that radiate the same signal on the same frequency. As such, a receiver in a SFN may receive signals with different delays that combine to form a kind of ‘unnatural’ additional multipath. Assuming that the range of delays of the multipath (natural or ‘unnatural’) do not exceed the designed tolerance of the system (i.e., slightly greater than the guard interval), all of the received signal components contribute usefully to a demodulated signal.
In general, multipath (natural and unnatural) interference can be viewed in the frequency domain as a frequency selective channel response. Another frequency-dependent effect for which COFDM offers benefits is when narrow-band interfering signals are present within the signal bandwidth. COFDM systems address frequency-dependent effects by implementing forward-error correcting coding. In general, the COFDM coding and decoding is integrated in a way which is tailored to frequency-dependent channels. Metrics for COFDM are slightly more complicated than those for OFDM. For example, when data is modulated onto a single carrier in a time-invariant system then all data symbols suffer from the same noise power on average. This requires that a decision process consider random symbol-by-symbol variations that this noise causes. When data are modulated onto multiple carriers, as in COFDM, the various carriers will have different signal-to-noise ratios (SNRs). For example, a carrier which falls into a notch in the frequency response will comprise mostly noise and a carrier in a peak will generally exhibit much less noise.
Another factor, in addition to the symbol-by-symbol variations, that should be considered in the decision process is that data conveyed by carriers having a high SNR are more reliable than those conveyed by carriers having low SNR. This extra a priori information is usually known as channel-state information (CSI). The CSI concept similarly addresses interference which can affect carrier selectively, just as noise does. In general, including CSI in the generation of soft decisions is the key to the performance of COFDM in the presence of frequency-selective fading and interference.
A satellite digital audio radio service (SDARS) system is one example of a SFN. As is well known, SDARS is a relatively new satellite-based service that broadcasts audio entertainment to fixed and mobile receivers within the continental United States and various other parts of the world. Within an SDARS system, satellite-based transmissions provide the primary means of communication and terrestrial repeaters provide communication in areas where the satellite-based transmissions may be blocked. As such, a given SDARS receiver may receive the same signal, with different delays from multiple transmitters. These delayed signals may form a kind of multipath interference. Today, Sirius satellite radio and XM satellite radio are two SDARS systems that are utilized to provide satellite-based services. These SDARS systems may provide separate channels of music, news, sports, ethnic, children's and talk entertainment on a subscription-based service and may provide other services, such as email and data delivery.
In these SDARS systems, program material is transmitted from a ground station to satellites in geostationary or geosynchronous orbit over the continental United States. The satellites re-transmit the program material to earth-based satellite digital audio radio (SDAR) receivers and to terrestrial repeaters.
In many situations, it would be desirable to provide secondary data, e.g., local or regional data, to a user of an SFN, such as an SDAR system. Unfortunately, as currently designed, SDAR systems are data bandwidth limited and are not capable of providing local or regional information, e.g., emergency broadcasting information, to a user of the SDAR system.
What is needed is a technique that allows an SDAR system to provide local or regional information to a user of the system.